With the recent spread of the copiers and printers which are based on electrophotography, these copiers and printers have come to be required not only to be energy-saving (diminish power consumption) mainly for the purposes of spread to domestic use and increasing the number of copier or printer functions, but also to be operated at a higher speed for the purpose of spread to the so-called gray area located between printing machines and copiers. There also is a desire for a copier or printer which can be operated at a lower rolling pressure for the purpose of fixing-roll simplification for attaining a machine cost reduction. In addition, since copiers having a double-side-copying function or equipped with an automatic document feeder have spread widely with the trend toward shifting to higher-grade copiers, the electrophotographic toners for use in such copiers and printers are required to have a low fixing temperature, to be less apt to cause offset, and to be excellent in the strength of fixing to a receiving paper so as to avoid smearing during both-side copying or in the automatic document feeder.
To meet the requirements described above, the following prior art techniques including a binder resin having an improved molecular weight or improved molecular weight distribution have been proposed.
Specifically, an attempt has been made to employ a binder resin having a reduced molecular weight to thereby attain a lower fixing temperature. However, the reduction in molecular weight has also resulted in a reduced viscosity besides the lowered melting point, and this has caused the problem of offset to the fixing roll. To avoid this offset phenomenon, a technique of widening the molecular weight distribution of the binder resin has been proposed. For obtaining a polyester resin having a widened molecular weight distribution, a technique of using a polyfunctional monomer having a functionality of 3 or higher as a crosslinking ingredient has been employed. However, this technique has a problem that the increased crosslink density results in an increased melt viscosity and impaired fixability, although effective in preventing the offset phenomenon. Another drawback is that the glass transition temperature (T.sub.g) of the resin should be lowered so as to impart sufficient fixability and this unavoidably impairs the storage stability of the toner. There is another technique for offset phenomenon prevention which comprises mixing a high-molecular polyester resin with a low-molecular polyester during melt kneading to thereby obtain a resin blend which has a widened molecular weight distribution for obtaining both anti-offset properties and fixability. This technique, however, has a drawback that since two resins having considerably different melt viscosities are kneaded together, a homogeneous dispersion comprising the two resins cannot be obtained and, hence, a combination of sufficient fixability and anti-offset properties is unable to be obtained.